Supercomputers and data centers are driving the need for high interconnect bandwidth as this generally results in more efficient use of microprocessors in real calculations. For distances greater than about 20 meters (m), electrical interconnects are impractical and optics are generally used for these longer rack-to-rack interconnects today. As costs of these optical interconnects come down they will take over even for board to board and eventually for on-chip interconnects. These optical interconnects, instead of running at ever increasing bit rates, tend to adopt parallel fiber architectures. This creates the need for multiple laser sources. One of the most attractive devices to meet this need is the VCSEL due to its low power consumption, small size and suitability for wafer test.
In the article “Fully Embedded Board-Level Guided-Wave Optoelectronic Interconnects” from the authors Ray T. Chen, et al., published in Proceedings of the IEEE, Vol. 88, No. 6, June 2000, an optical interconnect at board-level is described for coupling light from VCSELs into waveguides and then into silicon metal-silicon-metal (MSM) photodetectors. As is indicated in paragraph “III. Thin Film Waveguide Couplers”, this article attempts to solve the problem of surface-normal 1-to-1 board level coupling into waveguides. Chen, et al. do not disclose or teach silicon-level coupling and the surface-normal or perpendicular coupling under 90 degrees investigated in this article involves substantial optical losses.
Individual VCSELs or VCSEL arrays are mounted by direct or indirect attachment of the VCSELs' substrate to another substrate containing a waveguide. The substrate may be glass, silicon, indium phosphide (InP), gallium arsenide (GaAs), etc. VCSELs are typically flip-chip mounted, i.e. the VCSELs are vertically flipped in order to present laser light to a light coupling device, e.g. a planar waveguide, a mirror, an optical detector, a diffraction grating, etc.
WO 2009/141332 discloses an integrated photonics device for coupling of light between a VCSEL and a waveguide via a diffraction grating on a silicon integrated photonic circuit. Substantial vertical coupling between VCSEL and waveguides facilitates fiber mounting and reduces packaging costs.
Other prior art solutions wherein the light of a single VCSEL is coupled into a waveguide are described in United States Patent Application 2011/133,063 and the article “Chip-to-Chip Optoelectronics SOP on Organic Boards or Packages” from the authors D. Balaraman, et al. Apart from the fact that the light from only a single VCSEL is coupled into a waveguide and the problem of density is not addressed, these prior art solutions also implement surface normal coupling, i.e. perpendicular coupling under an angle of 90 degrees involving substantial losses.
As the number of VCSELs working as multiple sources in the same system is increasing, it is desirable to scale the size of the VCSEL. In particular as reductions in the VCSEL power consumption progress (due to both the shorter distances on which they are employed and improvements in VCSEL design), coupled with more efficient ways of dissipating the heat produced, it becomes even more desirable to move to smaller VCSELs.
Unfortunately the size of VCSELs or VCSEL arrays have not to date been scalable. This is because a way must be found to couple the light from the VCSEL array into an optical fiber. VCSEL arrays are today coupled to fiber either by placing a fiber array directly above the VCSEL array or alternatively using an array of mirrors to turn the light through 90 degrees. Sometimes the end of the fiber is micro-machined and processed to integrate the mirror into the fiber tip. In most cases coupling loss is improved with an array of lenses in the optical path. In all cases the pitch of the coupling device is determined by the pitch of the fiber. Today the industry standard is 250 micrometers (um) although slightly smaller pitch is available as custom specials.
In U.S. Pat. No. 6,829,286 entitled “Resonant Cavity Enhanced VCSEL/Waveguide Grating Coupler” and in the article “Highly-Integrated, VCSEL-Based Optoelectronics for Fault-Tolerant, Self-Routing Optical Networks”, the inventors/authors P. S. Guilfoyle, et al. describe an optical coupler for coupling light from an array of VCSELs into an array of optical fibers. The optical coupler from Guilfoyle, et al. comprises two mirrors (see for instance Col. 2, ln. 4, and claim 1 of U.S. Pat. No. 6,829,286) and a surface normal grating structure (see for instance Col. 2, ln. 2, FIG. 2-8, FIG. 34-35, FIG. 39-41A, claim 11 of U.S. Pat. No. 6,829,286) in the resonant cavity between the two mirrors to couple the light perpendicularly into or out of a waveguide. The optical coupler disclosed in U.S. Pat. No. 6,829,286 is rather complex since it requires two mirrors, and its coupling efficiency remains limited since it implements perpendicular coupling through a surface-normal grating structure coupling the VCSEL light into a waveguide under an angle of 90 degrees which unavoidably involves substantial losses. As is indicated in Col. 2, ln. 28-32, U.S. Pat. No. 6,829,286 also makes use of multi-mode VCSELs further reducing the efficiency since such multi-mode VCSELs are consuming excessive power.
Until relatively recently, the size of certain Integrated Circuits (ICs) were limited by the size of their bond pads. The bond wires used to connect to the IC require a minimum size bond pad and a minimum spacing to adjacent wires and they are typically located around the edge of the IC to minimize bond wire length. The advent of silicon interposers with redistribution layers and Through Silicon Vias changed this. IC's could be designed with bond pads and bumps (significantly smaller than bond pads for wire bonding) and the bond pads could be placed anywhere on the chip. The IC would then be flip-chipped onto the silicon interposer which redistributes or spreads out the signals to larger pads on the opposite side of the interposer. In this way, the size of the expensive IC is minimized.
In the same way as the size of IC's have been limited by the area required to get the electrical signals off the chip, the size of VCSEL arrays is limited by the area required to receive the light output from the VCSEL.
It is an objective of the present invention to disclose an arrangement to couple the light from an array of VCSELs into waveguides and then route the light to output ports which can be interfaced to a standard fiber array that overcomes the technical problems of the above-identified existing solutions. More particularly, it is an objective to disclose a silicon-level optical coupler between VCSELs and optical waveguides that is highly reproducible and has increased efficiency in comparison with existing solutions in terms of reduced power consumption, higher achievable density and reduced optical coupling losses.
It is another object of the present invention to disclose an improved arrangement for coupling light from multiple VCSELs arranged in arrays, emitting light perpendicular or nearly perpendicular to the plane of the chip, into waveguides integrated in the optical interposer using diffraction grating couplers.